Obesity and overweight affects most Americans, but there are some who resist obesity, just as some animals resist obesity. Animal and human studies indicate that spontaneous physical activity (SPA), which generates nonexercise activity thermogenesis (NEAT), is an important defense against weight gain. Brain mechanisms control the level SPA, and the NEAT so generated, but the specifics are undefined. We propose that hypothalamic orexin A (OxA) is an important part of the SPA regulatory pathway, and that sensitivity to OxA signals enhancing SPA and NEAT helps determine propensity for obesity. The objectives of this application are to test the following hypotheses: (1) elevated OxA mediated SPA protects against weight gain and (2) increased OxA-mediated SPA in obesity resistant (OR) rats results from increased orexin receptor expression. Our long-range goal is to use knowledge of SPA and NEAT regulatory pathways to guide therapeutic interventions on these mechanisms among obese humans. We plan 3 aims: Aim 1) Characterize orexin neuron subpopulations important to SPA. A) Are there strain differences in orexin message and peptide in subpopulations of orexin neurons? B) Does destruction of subpopulations of orexin neurons affect SPA? Aim 2) Determine importance of OxA-mediated SPA to weight gain. A) Does reduced OxA action in rLH of OR rats reduce resistance to weight gain? B) Does SPA induced by high dose OxA stimulation of rLH reduce weight gain in OP rats? Aim 3) Verify that increased orexin sensitivity in OR rats is due to increased orexin receptor expression. A) Is orexin receptor number increased in OR compared to OP rats? B) Is c-fos activation in rLH orexin receptor bearing neurons higher in OR than OP rats? C) Does reducing orexin receptor bearing neurons in rLH of OR rats enhance weight gain? A greater understanding of brain orexin and its relationship to SPA in obesity will fill a large gap in knowledge of centrally mediated SPA and NEAT, and its importance to obesity resistance. PUBLIC HEALTH RELEVANCE The current proposal is aimed at understanding the role of a specific brain pathway that controls physical activity, which is important to body weight regulation. Obesity is a major health problem affecting at least one third of the U.S. population, and is a risk factor for several diseases, including heart disease, diabetes and several types of cancer. Underlying part of the problem is disordered regulation of physical activity, and a better understanding of the brain pathways important to this is necessary in developing treatments for obesity.